The present invention relates in general to battery monitoring in electric vehicles, and, more specifically, to synchronizing measurements made by separate modules for different cells in a multi-cell battery or battery packs.
The DC power source (e.g., a battery) and other elements of electric drives for full electric vehicles or hybrid electric vehicles require monitoring in order to maximize efficiency and performance as well as to detect potential malfunctions. Common battery types such as lithium ion (Li-Ion) use a large number of individual battery cells stacked together into a battery pack. Besides monitoring the total voltage output by a battery pack, each cell is typically monitored individually to determine their voltage production, current, and other parameters. The temperature of each cell is also monitored in order to protect against overheating.
It is very challenging to reliably monitor the various battery conditions because of the high-voltage levels involved, the range of intermediate voltages at which respective cells operate within the stack, and the high levels of accuracy required. Various battery monitoring integrated circuit devices have been developed commercially for use in the vehicle environment. Examples of a commercially available battery monitoring IC device include the AD7280A device available from Analog Devices, Inc., of Norwood, Mass., the LTC6804 devices available from Linear Technology Corporation of Milpitas, Calif., and the ISL94212 Multi-Cell Li-Ion Battery Manager available from Intersil Corporation of Milpitas, Calif.
A typical Battery Energy Controller Module (BECM) uses an IC device that includes or can be programmed to include various battery management and communication functions in addition to the monitoring functions. Due to space limitations, however, a BECM device typically includes inputs for monitoring no more than about a hundred individual battery cells. A single battery pack can have over one hundred cells, and some vehicles may include more than one battery pack. Therefore, a Battery Pack Sensing Module (BPSM) is often used with additional monitoring ICs to measure the parameters for other cells and report them to the BECM.
In order to accurately measure and evaluate various parameters associated with the individual battery cells, it becomes important to synchronize the measurements taken for the plurality of cells. Due to changes in load, for example, the individual cell voltages and currents may change very quickly so that measurements made at different sampling times diminish the ability to accurately compare performance of one cell against another. Each monitoring IC samples all of its cells in parallel at one time, referred to as the sampling moment.
Since each BPSM contains an internal time reference, the measurements for its cells are easily synchronized. When multiple ICs are required due to a high number of cells in the battery pack(s), however, coordinating signals between the separate BPSM s may be necessary. As suggested in US Patent Application Publication 2010/0161260 A1, an externally generated timing pulse can be distributed to each of the BPSM s so that sampling of the battery cell parameters can be triggered simultaneously. This manner of synchronization has the disadvantages of not being able to provide advance notification of a measurement to allow any preparatory steps to be taken in an IC, and any failure to generate or detect the timing signal may go undetected and missing measurements may result.
Another solution has been to allow a BECM to generate its sampling moment internally and then send a multiplex message to the BPSM upon the occurrence of the sampling moment. Multiplex messages, however, include inherent delays in message arbitration and transmission that may create undesirable delays between the primary sampling moment of the BECM and the secondary sampling moment of the BPSM.